The present invention relates to semiconductor mask-to-wafer alignment techniques and more particularly, to an alignment method and apparatus that utilizes the interference and diffraction effects produced by coherent light impinging upon or passing through repetitive patterns on the mask and wafer.
The sequential processing operations used in the manufacture of semiconductors require accurate alignment of a photomask to the semiconductor wafer. In the early years of semiconductor production, the alignment step was performed manually by using high power microscopes to view the relative alignment of target patterns formed on both the mask and wafer. As the semiconductor industry matured and production capacity increased the alignment operation was automated by means of various electro-optical techniques. At the time, these techniques provided the required degree of alignment accuracy. However, recent advances in the semiconductor industry have produced a significant increase in circuit densities with a concomitant tightening of the allowable alignment tolerances between the mask and the wafer.
It is accordingly a general object of the present invention to provide an improved method and apparatus for fine aligning a photomask and a semiconductor wafer.
It is a specific object of the invention to provide a coherent light alignment method and apparatus that utilizes the interference and diffraction effects produced by coherent light interacting with repetitive alignment patterns on the mask and the semiconductor wafer.
It is still another object of the invention to use the resultant interference information to obtain phase dependent electrical signals for controlling conventional X, Y and .theta. positioning mechanisms for the mask and/or wafer.
It is a feature of the invention that this method can be implemented with conventional available mechanical, electrical and optical components.
It is another feature of the invention that the apparatus thereof can be retrofitted to existing automatic mask-to-wafer aligners in order to increase the alignment accuracy of these aligners.